1. Field of the Invention
The present invention is generally related to pulse-width-modulated converters, and more particularly to two new classes of pulse-width-modulated converters that provide constant frequency operation, low-switching losses, and low-component stress; one class is a novel-zero-voltage transition converter and the other is a novel zero-voltage-switching converter.
2. Description of the Prior Art
Those skilled in the art have recognized the benefits and desirability of operating pulse-width-modulated converters at high frequency, particularly, direct current to direct current converters. Basically, high-frequency operation is desirable as it allows a size and weight reduction of the converter for a given power. However, switching losses, component stresses, and noise due to parasitic oscillations are inherent problems with pulse-width-modulation technology, and these problems have limited, as a practical matter, the operating frequency of pulse-width-modulated converters. Zero-voltage-switching, quasi-resonant converter technology can eliminate much of the switching losses and parasitic oscillations associated with the power switches in a converter. However, prior art zero-voltage-switching, quasi-resonant technology have additional limitations in that they produce a high voltage stress in certain components, have a limited range of loads with which they can operate, and produce a variable switching frequency. For example, in a zero-voltage-switching, quasi-resonant converter (ZVS-QRC) the power switch in a single-ended ZVS-QRC suffers from excessive voltage stress proportional to the load range. Using the buck ZVS-QRC as an example, for a 10% to 100% load range, the peak voltage stress of the power switch can be 11 times the input voltage. Therefore, high voltage MOSFET(s) accompanied with high on-resistance and large input capacitance have to be used, increasing the conduction loss and the gate driver loss. In addition, a wide switching frequency range is required for a ZVS-QRC to operate with a wide input voltage and load range. The wide frequency range makes optimization of the power transformer, input/output filters, control circuit, and power switch drive circuit difficult. For example, to decrease conduction loss, power MOSFETs with low on-resistances are preferred. However, MOSFETs with low on-resistance are accompanied by large input capacitances, which can cause significant driver loss at high-frequency operation, especially at high line and light load.